Resolution of nitrogen base



Patented Dec. 21, 1948 RESOLUTION OF NITROGEN BASE MIXTURES Karl Henry Engel, Teaneck, N. J assignor to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York No Drawing. Application November 29, 1943, Serial No. 512,241

' Claims. 1

This invention relates to the resolution of mixtures of heterocyclic nitrogen bases containing 3- and 4-picolines and other pyridine homologs, particularly to the separation of a substantially pure picoline product from such mixtures.

The picolines and other homologs of pyridine are customarily extracted along with other nitrogen bases from coke oven distillates and other sources of heterocyclic nitrogen bases by means of an aqueous solution of mineral acid, usually sulfuric acid. After liberation from these acid solutions by means of alkali, the base mixtures thus obtained are customarily fractionally distilled to obtain fractions predominating in a particular base. Bases such as pyridine, boiling point 116 C., and 2-picoline, boiling point 129.4 C., can generally be obtained in reasonably pure form by fractional distillation, but for the most part the remaining compounds cannot be separated by ordinary distillation methods alone. Instead, the fractions obtained, even when they are of relatively narrow boiling range, contain increasing numbers of pyridine homologs and their isomers. Mixtures of this kind have found limited practical application, mainly as special solvents and are of relatively low economic value. The individual bases in pure form, however, are products of considerable commercial interest, being useful, for example, in the preparation of pharmaceutical products and dyes. The purity requirements in these fields are quite rigid,

0f especially high potential value are the nitrogen bases in the fractions having a boiling range within 140-148 C'. When mixtures "of heterocyclic nitrogen bases are recovered from most types of coal tar and other sources of these bases, the fractions taken within the range 140-148" C., even closely cut fractions within this boiling range, in general, contain as components three pyridine homologs of substan'- tially the same boiling point in roughly equal proportions; i. e., 2,6-lutidine, boiling point 143.8 C., B-picoline (beta-picoline), boiling point 143.8 C., and i-picoline (gamma-picoline), boiling point 144.8 C. Derivatives of several of these compounds are becoming of increasingly greater importance. Nicotinic acid, for example, a member of the vitamin B complex, may be produced by oxidation of 3-picoline.

Since the three components making u the usual nitrogen base fractions boiling in the range Lin-148 C. (i. e. 2,6-lutidine, 3-picollne and 4- picoline) have substantially the same boiling points, fractional distillation is obviously unsatisfactory as a means for resolving the mixtures.

Other methods of resolving such mixtures heretofore described in the published prior art have been generally unsatisfactory in regard to products, yields, and production costs. Such other methods have usually depended upon fractional crystallization of the more common salts of the bases present, e. g., the sulfates, oxalates, chlorates, picrates, ferrocyanides, and the like. Isolation or purification of individual bases by such methods has been found to be extremely cumbersome and generally insufiiciently sharp to obtain compounds of a purity demanded in industry. These methods also have often presented other drawbacks such as prohibitive cost, explosion hazards, and toxicity.

It is an object of this invention to provide a new and commercially attractive process for resolving nitrogen base mixtures which contain 3-picoline or 4-picoline, or both, along with likeboiling bases such as 2,6-lutidine.

It is a particular object of this invention to provide a process for separating a substantially pure picoline product from tar base fractions boiling in the range -148 C. containing such closely-boiling nitrogen bases as 3-picoline, 4- picoline and 2,6-lutidine.

I have discovered that a high quality picoline product may be recovered from mixtures of 3- picoline or 4-piccline or both with other pyridine homologs not separable from these pico1ines by distillation, e. g. 2,6-lutidine, by treating the mixture with zinc chloride to form zinc ;'chloride addition compounds of bases in the mixture, and thereafter subjecting the resulting mixture to distillation, preferably steam distillation, to remove bases other than 3- and 4-picolines as distillate. I have found that whereas the 3- and 4-picolines form stable zinc chloride addition compounds. close-boiling, closely related bases such as 2,6- lutidine surprisingly form zinc chloride addition compounds sufficiently unstable that they decompose readily when heated and such bases may be substantially completely removed by distillation without substantially aifecting the 3- a'nd 4-picoline addition compounds.

I have further found that when a mixture containing 2,6-lutidine and 3- or 4-picolines or both is treated with zinc chloride and the resulting mixture distilled, as above described, and a highly-enriched or nearly pure 2,6-lutidine is recovered from the distillate, from such 2,6-lutidine-enriched product a-substantially pure 2,6- lutidine may be separated by repeating the procass of my invention, as above described, on this material.

and from other natural, as well as synthetic,.

sources of these base mixtures containing substantial amounts of 3- or 4-picoline and other nitrogen bases such as 2,6-lutidine not readily separable from the picolines by distillation. The mixture from which a picoline product, and if desired a 2,6-lutidine product, is separated in accordance with my invention is most advantageously a close-cut fraction of heterocyclic nitrogen bases (e. g. of coke-oven origin), boiling in the range 140-148 C., containing as its principal components 3-picoline, 4-picoline and 2,6- lutidine. My invention may thus comprise fractionating a broader mixture of heterocyclic nitrogen bases containing one or both of these picolines along with close-boiling bases to ob tain such a close-cut fraction, i. e. a fraction boiling predominantly within the range 140-148" C., and recovering a substantially pure picoline product and, if desired, a substantial y pure 2,6-lutidine product, from this fraction by forming the zinc chloride addition product of the picolines.

My invention further comprises a process in which separation of a 3- and 4-picoline product, as above described, is combined with steps for separating the 3-picoline from the 4-picoline. Thus my invention is applicable to resolving into its components a mixture of 3-picoline, 4-picoline and 2,6-lutidine.

In a preferred method of carrying out the process of my invention, a mixture of 3-picoline, 4- picoline and 2,6-lutidine, as above described, is treated with zinc chloride. The zinc chloride is preferably employed in concentrated aqueous solution, and although a larger amount of zinc chloride is operative in the process preferably an amount not substantially greater than that necessary for reaction with the 3- and 4-picolinespresent is employed, and most advantageously I employ approximately the stoichiometric amount of zinc chloride for reaction with the 3- and 4- picolines to form addition compounds in which two mols of picoline are combined with one mol of zinc chloride. Any convenient working temperature, up to about 50 C., e. g., the normally prevailing room temperature may be employed in reacting the zinc chloride with the nitrogen bases.

After treatment with the zinc chloride reagent,

the lutidine is substantially completely removed.

Steam distillation by distillation of the mixture. is much preferred for effective separation; it is believed that the presence of water at distillation temperatures exerts some influence such as pref erential hydrolysis on zinc chloride-nitrogen base addition components other than the 3- and 4- picoline addition compounds, aiding in the separation of the bases.

The steam distillation is conveniently carried out at atmospheric pressure. Under these conditions the zinc chloride addition compounds of the 3- and 4-picolines form a separate liquid phase which can readily be agitated. As an alternative procedure, the 3- and 4-picoline addition compounds may be induced to crystallize and the removal of the 2,6-lutidine and other bases may be carried out by evaporation or steam distillation of the charge at temperatures below the fusion point of the addition compounds, generally 4 below about 70 or 80 0., under slightly reduced pressures.

Separation by means of steam distillation as above described furnishes a residual 3- and 4- picoline product, nearly free of 2,6-lutidine. The bases in the steam distillate may vary somewhat in composition, depending on how long distillation was continued, or on the relative proportion of steam applied to a given charge. Bases obtained from the distillate will generally contain from about 70-to 85% 2,6lutidine. Such distillates may be subjected to further zinc chloride treatment, in each case preferably adding the required amount of zinc chloride to react with the picolines, and again steam-distilling.

The picoline-zinc chloride product recovered as distillation residue may be treated to liberate the picoline; for example, by agitation with an aqueous solution of an alkali such as sodium hydroxide 01' sodium carbonate. The zinc oxide or carbonate which precipitates in these base liberation operations may be reconverted to zinc chloride by adding the stoichiometric quantity of hydrochloric acid and may then be reused in the process.

A 3'- and 4-picoline mixture may thus be recovered and may be furtherresolved by precipitating 3-picoline in the form of its phosphate, as disclosed in my copending application Serial No. 346,347, filed July 19, 1940, now Patent No. 2,408,975, dated October 8, 1946, or may be further resolved by precipitating 4-picoline in the form of its calcium chloride addition compound, this step, if desired, being combined with phosphate precipitation of the 3-picoline, as disclosed in copending application Serial No..44l,557, filed May 2, 1942, now Patent No. 2,336,502, dated December 14, 1943.

' When a lutidine concentrate is obtained, for example by the first steam distillation as above described, a pure lutidine product may be precipitated therefrom as phosphate, if desired, in accordance with the process of my copending application Serial No. 346,347 above referred to, instead of treating the lutidine concentrate again by the zinc chloride process of my present invenion.

The following examples are illustrative of the process of my invention:

Example 1 The material used in this example consisted of a mixture of heterocyclic bases containing mainly 2,6-lutidine, 3-picoline and 4-picoline. Its boiling range (bulb distillation) was 140.4-144.1 C.; specific gravity was 0.946 at 25 C. It had the following approximate composition:

Per cent 2,6-1utidine 34 3-picoline 29 4-picoline and other bases 37 An iron kettle equipped for agitation was charged with 300 parts by weight of this mixture of bases. 300 parts of a zinc chloride solution containing 50% of zinc chloride were added, the zinc chloride being calculated to combine with approximately the amount of 3- and 4-picolines present.

The charge was heated to the boiling point of Water, the zinc chloride-picolines compound remaining molten at this temperature. The agitated mass was steam distilled to remove the bulk of the 2,6-lutidine, together with a smaller proportion of the isomeric picolines. A total of approximately 450 parts of distillate was obtained. This material was admixed with an equal volume of saturated salt solution, and the salted out layer of bases drawn ofi. The sepa rated bases, completely dehydrated and distilled, amounted to 110 parts by weight, and contained 85% of 2,6-lutidine. Steam distillation was continued, removing approximately 20 parts of additional bases, of lower 2,6-lutidine and higher picoline content.

To the kettle residue was added 92 parts by weight of a 50% solution of sodium hydroxide, liberating the picolines. These were removed by steam distillation; they Were salted out of the aqueous condensate and dehydrated, 112 parts by weight being obtained. These bases were found to contain:

Per cent 2,6 -lutidine 5 3-picoline 47 4-pic0line 43 Example 2 This example illustrates a second zinc chloride treatment in accordance with the process of my invention to obtain substantially pure 2,6- lutidine from a 2,6-lutidine concentrate produced as in the preceding example:

A fraction of heterocyclic bases enriched in 2,6-lutidine, as obtained by the method described in Example 1, had the following composition:

Per cent 2,6-lutidine 78 3-picoline 9 4-picoline 13 Two hundred parts by weight of this mixture and 40 parts of zinc chloride dissolved in 300 parts of water were mixed together and steam distilled until approximately 340 parts by weight of distillate had been collected. This distillate was saturated with sodium chloride, 2,6-lutidine separating out as an upper layer. The material was isolated, dehydrated with dry sodium hydroxide and distilled for color, 136 parts by weight being obtained. The material contained 94% 2,6-lutidine.

The distillation residue consisting of zinc chloride addition compounds and water can be worked up for free heterocyclic bases and zinc chloride. Alternatively, the residue may be added to a fresh mixture of bases in an operation similar to that described in Example 1, the residue serving here mainly as a partial source of zinc chloride.

Since certain changes may be made in carrying out the above process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. In a process for separating a picoline product from a mixture of at least one of the bases 3-picoline and 4-picoline with 2,6-lutidine, the steps that comprise treating the mixture With zinc chloride to form picoline-zinc chloride addition product, and subjecting the resulting mixture to steam distillation to remove as distillate said 2,6-lutidine.

2. In a process for separating a highly enriched picoline product from a mixture of at least one of the bases B-picoline and 4-picoline with 2,6-lutidine, the steps that comprise treating the mixture with'approximately the stoichiometric amount of zinc chloride, in aqueous solution, to form picoline-zinc chloride addition product, subjecting the resulting mixture to steam distillation to remove said 2,6-lutidine substan tially completely as distillate, and treating the residual picoline-zinc chloride addition product with an aqueous solution of an alkali to decompose the picoline-zinc chloride addition product and liberate therefrom a highly enriched picoline product.

3. In a process for separating a highly enriched picoline product from a tar base material containing at least one of the bases 3-picoline and 4-picoline and 2,6-lutidine, the steps that comprise fractionally distilling the tar base material to obtain a fraction boiling in the range 148 C. predominating in the above-specified picoline and 2,6-lutidine, treating the mixture with zinc chloride to form picoline-zinc chloride addition product, subjecting the resulting mixture to steam distillation to remove substantially completely as distillate said 2,6-lutidine, and treating the residual picoline-zinc chloride addition product with alkali to liberate therefrom a picoline product substantially free from other nitrogen bases.

4. In a process for separating substantially pure lutidine and picoline products from a mixture of at least one of the bases 3-picoline and 4-picoline with 2,6-lutidine, the steps that comprise treating the mixture with approximately the stoichiometric amount of zinc chloride to form picoline-zinc chloride addition product, subjecting the resulting mixture to steam distillation until substantially all 2,6-lutidine has been removed as distillate, treating distillate base again with approximately the stoichiometric amount of zinc chloride to form picoline-Zinc chloride addition compound, and steam distilling this mixture to separate a substantially pure 2,6-lutidine product as distillate.

5. In a process for resolving into its components a mixture of 3-picoline, 4-picoline and 2,6-lutidine, the steps that comprise treating the mixture with approximately the stoichiometric amount of zinc chloride to react with the picolines substantially completely to form picoline-zinc chloride addition product, subjecting the resulting mixture to steam distillation to remove said 2,6-lutidine substantially completely as distillate, decomposing the residual picoline-zinc chloride addition product to liberate therefrom a picoline mixture substantially free from other nitrogen bases, and precipitating B-picoline as phosphate from said picoline mixture.

KARL HENRY ENGEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,750,082 Wyler Mar. 11, 1930 2,035,583 Bailey Mar. 31, 1936 2,263,912 Bailey et al Nov. 25, 1941 2,309,324 McAllister et al. Jan. 26, 1943 2,336,502 Reimers Dec. 14, 1943 2,374,418 Cislak Apr. 24, 1945 OTHER REFERENCES J. Chem. Soc. vol. 95, pages 668-685 (Cited in Chem. Abstracts, vol. 3, page 2079).

Jour. Amer. Chem. Soc. vol. 43 (1921) pages 1936-1940.

Chemical Abstracts 1922, page 97.

Chemical Abstracts, vol. 3, page 2079. 

